078 Mechanical Strain Induces Hypertrophic Scars in Mice by Reducing Stromal Cell Apoptosis

Intro: Pathophysiology of hypertrophic scar formation remains unclear, potentially involving mechanical strain, burns, or infection. We sought to specifically examine the role of mechanical strain in hypertrophic scars. To do so, we developed and characterized a novel murine model which uses mechanical strain to produce hypertrophic scars in mice. Methods: Paired incisions were created on C57BL6 mice (n = 20). After closure, mechanical strain was applied across one wound using a novel device; the other wound was a control. Starting at day 3, mechanical strain was increased every other day over 4 wks. Wounds were harvested each week, and examined histologically using Sirius red, DAPI, BrdU for proliferation, and caspase 3 for apoptosis. Results: Strained wounds showed features of hypertrophic scars: raised borders, loss of rete pegs and adnexal structures in the epidermis overlying scars, blood vessels that were perpendicular and fibrillary collagen that was parallel to skin surface. Features persisted beyond 6 months. Wound collagen deposition in strained wounds increased 6-fold at 1 wk and over 12-fold at 2–4 wks, in parallel to an increase in number of stromal cells within scar (28-fold). This resulted in an increase in number of cells per unit area of collagen (over 42%). In all wounds, there was no significant difference in proliferation. There was a significant reduction in blood vessel (4-fold) and fibroblast (3-fold) apoptosis at 1–4 wks in strained wounds. Conc: Mechanical strain alone is sufficient to produce hypertrophic scars in this model that are indistinguishable from human scars. Notably, hypertrophic scars appear to result from a marked reduction in apoptosis, rather than from a significant proliferation and upregulation of cellular collagen deposition, validating previous in vitro studies.